Solar modules which convert the light of the sun directly into electric energy enable natural resources to be utilized for power generation.
The most important constituent of solar modules are solar cells by means of which the direct conversion of sunlight into electric energy is effected. The solar modules are frequently used outdoors, e.g. on buildings. The solar cells present in the solar modules therefore have to be protected against environmental influences. Since moisture penetrating into the solar cells can greatly shorten the life of the solar cells and thus of the solar module as a result of corrosion, durable encapsulation (embedding) of the solar cells is of particular importance. The material used for encapsulation of the solar cells has to be transparent to sunlight and at the same time make it possible for the solar modules to be produced inexpensively. Material which has frequently been used in the prior art for embedding the solar cells are ethylene-vinyl acetate copolymers.
For example, WO-A-97/22637 relates to an essentially transparent and colourless embedding material which is free of absorber components for ultraviolet light and is made up of a polymer component and a crosslinking reagent. According to WO-A-97/22637 ethylene-vinyl acetate copolymer is preferably used as polymer component. In Example 1 in WO-A-97/22637, a random ethylene-vinyl acetate copolymer made up of 67% by weight of ethylene and 33% by weight of vinyl acetate is used.
EP 1 164 167 A1 discloses encapsulation materials comprising an ethylene-vinyl acetate copolymer (EVA), a crosslinker and a polymerization inhibitor. According to EP 1 164 167 A1, the EVA has a vinyl acetate content of from 5 to 50% by weight. At contents above 50% by weight, the mechanical properties of the EVA deteriorate and it becomes difficult to produce EVA films, according to EP 1 164 167 A1. EP 1 164 167 A1 gives no information about the method of producing the EVA used in EP 1 164 167 A1. An EVA copolymer containing 26% by weight of vinyl acetate is used in Examples 1 and 2 in EP 1 164 167 A1.
EP 1 184 912 A1 likewise relates to an embedding material for solar cells which is made up of an ethylene-vinyl acetate copolymer (EVA). According to EP 1 184 912 A1, the vinyl acetate content of the EVA copolymer is from 10 to 40% by weight. Contents of more than 40% by weight of vinyl acetate are unfavourable according to EP 1 184 912 A1 since EVA copolymers having contents of more than 40% by weight of vinyl acetate flow easily and thus make the embedding process for the solar cells more difficult. Furthermore, according to EP 1 184 912 A1, EVA copolymers having a vinyl acetate content of more than 40% by weight are sticky so that the EVA film used for embedding is difficult to handle. The EVA films which are preferably used according to EP 1 184 912 A1 are crosslinked.
JP-A 2003-051605 discloses a film for a solar module which is made up of an EVA copolymer which is mixed with an organic peroxide, a silane coupling reagent and stabilizers. The vinyl acetate content of the EVA copolymer according to JP-A 2003-051605 is 27% or more. In the examples, an EVA copolymer having a vinyl acetate content of 28% is used.
JP-A 2003-049004 relates to a flexible film which is suitable for the embedding of solar cells without crosslinking. The flexible film is preferably made up of an ethylene polymer or an ethylene-α-olefin copolymer or an ethylene-acrylate copolymer. According to the description in JP-A 2003-049004, alternative systems to EVA systems should be made available.
Proceeding from the prior art, it is an object of the present invention to provide solar modules or embedding materials for solar cells which display a long life, excellent UV resistance and simplified fixing of the solar cells.
This object is achieved by a solar module containing at least one layer made up of at least one α-olefin-vinyl acetate copolymer.